Buprenorphine is a low efficacy opioid analgesic with a favorable side effect profile. Although a powerful analgesic in both rodents and humans, it causes little respiratory depression, induces few abstinence symptoms, and it is very long lasting. These characteristics have made this compound suitable not only as an analgesic, but also for heroin abuse pharmacotherapy. Considerable evidence has suggested the involvement of the orphan receptor from the opioid receptor family, NOP receptor (formerly called ORL1), and its endogenous ligand N/OFQ (formerly nociception/Orphanin FQ), in opioid actions with respect to antinociceptive activity, tolerance development, and reward. Recent evidence suggests that some of the behavioral properties of buprenorphine are due to binding to NOP receptors at high concentrations. This suggests that other mixed NOP/mu receptor compounds, with different affinity profiles, may also have antinociceptive activity with low respiratory depression and low abuse potential. In the following application, experiments will be conducted in knockout (KO) mice to test the hypothesis that NOP receptors play an integral role in the antinociceptive activity of buprenorphine and SR16435, a mixed NOP/mu receptor ligand developed at SRI. In Specific Aims 1 and 2, buprenorphine and SR16435, respectively, will be examined for antinociceptive activity in mu, NOP, and ppN/OFQ KO mice and their respective wild types. Tail flick will be used to determine the involvement of the receptors or the peptide in the actions of buprenorphine and SR16435 in acute antinociceptive activity. The effectiveness of these compounds in the Chronic Constriction Injury (CCI) model of chronic/neuropathic pain will also be determined. Following CCI surgery, the effects of buprenorphine and SR 16435 will be determined on mechanical allodynia using vonFrey filaments and thermal hyperalgesia using the tail flick assay in each of the KO mouse strains. Specific Aim 3 will be used to synthesize and characterize novel mixed NOP/mu receptor ligands. New buprenorphine analogs with higher NOP affinity will be synthesized at the University of Bath, and characterized in vitro for binding and functional activity. Promising buprenorphine analogs, along with NOP/mu receptor compounds, previously synthesized at SRI, will be tested for antinociceptive activity in the tail flick assay in wild type mice. Potent analgesics will also be tested for rewarding properties using the place conditioning procedure in mice.